This invention relates generally to VAR generators and it relates in particular to high speed circuits for ascertaining the firing angles in static VAR generators.
Static VAR generators compensate for the effects of highly reactive loads such as arc furnaces on the terminals of an AC supply system. The static VAR generator or flicker compensator control circuit measures the magnitudes of the arc furnace currents in consecutive half cycles. This information is used to compute necessary compensation currents in terms of subsequently required firing angle signals for the thyristors of the static VAR generator. The VAR generator output current can only be adjusted once during each half cycle per phase. Consequently the response time of the flicker compensator and thus its effectiveness for flicker reduction is largely dependant upon the speed with which the arc furnace current is measured and converted to a firing angle. U.S. Pat. No. 3,597,518 entitled "Electric Arc Furnace Control" by R. W. Roberts, issued August 3, 1971 teaches a control system for an electric arc furnace which is designed to maintain constant arc peaks. The system is designed to respond quickly to large impedance errors, is insensitive to short-term impedance fluctuations, and at the same time is rarely highly sensitive to continuing small errors. U.S. Pat. No. 3,728,516 entitled "Welding Power Source" by A. D. Daspit, issued Apr. 17, 1973 teaches the concept of controlling SCR's in response to current and voltage applied to an electric welding device in which a capacitor is periodically semi-discharged to a switching device such as an SCR. It would be advantageous to provide a control device for a static VAR generator which very quickly senses and computes the parameters necessary to set the firing angles of the thyristors in the static VAR generator and thereafter sets those angles almost immediately after making the computation. It would be preferable if this detection and setting for firing could be done within one half cycle. In copending application Ser. No. 535,918, now Pat. No. 4,000,455, apparatus and method are taught for accomplishing the above-mentioned purpose. Furthermore the calculation takes place in at least one of the embodiments of the aforementioned application immediately prior to the half cycle in which the results of the computations are utilized. It would be advantageous if the time for completing the necessary calculations actually extended into the half cycle in which they were utilized.